Method for preparing perovskite complex oxide powder of formula ABO3

ABSTRACT

Ba(OH) 2 .8H 2 O is fused by heating. The fused Ba(OH) 2  is allowed to react with TiO 2  powder having a specific surface area of 250 m 2 /g or more to prepare a cubic crystalline BaTiO 3  having high crystallinity. The BaTiO 3  is calcined to yield a fine, tetragonal crystalline BaTiO 3  powder having high crystallinity. Thus, a high quality BaTiO 3  having high crystallinity can be prepared at a low cost.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to methods for manufacturing a complexoxide powder and complex oxide powders, and particularly to a method formanufacturing a complex oxide powder used for electronic materials, suchas dielectric ceramics, and a complex oxide powder prepared by themethod.

2. Description of the Related Art

Monolithic capacitors have recently been miniaturized with theircapacitance becoming larger, and accordingly, the thickness ofdielectric elements has become smaller. In order to manufacture such alaminated monolithic capacitor, the crystal grains of dielectricceramics need to be small and the crystal growth of the grains must besuppressed. Accordingly, complex oxides constituting the ceramics arerequired to have a small grain size and high crystallinity.

Barium titanate (BaTiO₃) powder has been typically used as such acomplex oxide, and a wet synthesis (Kubo, K. et al., “Wet process forsynthesizing barium titanate”, Journal of Chemical Industry by TheChemical Society of Japan, Vol. 71, No. 1, 114-118 (1968)) is widelyknown as a method for preparing BaTiO₃ powder.

In this method, barium hydroxide octahydrate (Ba(OH)₂.8H₂O) and atitanium oxide (TiO₂) gel containing about 95% of water are mechanicallytriturated and mixed to react with each other while carbon dioxide isremoved. The resulting precipitate is separated from the reactionproduct. Then, acetic acid (6 N) is added to the precipitate, followedby heating to 50° C. Unreacted Ba(OH)₂ and the by-products BaCO₃ andBa₂TiO₄ are dissolved and extracted. The remaining precipitate is washedwith water and dried to yield the BaTiO₃.

As for another method, a hydrolysis method is disclosed in JapaneseExamined Patent Application Publication No. 3-39014. In this method, aproduct from hydrolysis of a titanium compound and a water-solublebarium salt are allowed to react with each other in a strong alkalineaqueous solution to yield BaTiO₃ fine particles. Specifically, thispublication describes creating a product from the hydrolysis of atitanium compound, such as TiCl₄, and Ba(NO₃)₂, in a strong alkalineaqueous solution, such as KOH or NaOH of pH 13.0 or more, to yield cubiccrystalline BaTiO₃.

A pulverization method, hydrothermal synthesis, sol-gel method, andalkoxidehydroxide route are also known as methods for preparing BaTiO₃powder.

The pulverization method is suitable for preparing a fine particlematerial at a low cost. In the pulverization method, a mixture of BaCO₃and TiO₂ is mechanically pulverized and uniformly mixed using a ballmill, a sand mill, or the like. The resulting fine particles are driedand calcined to yield the BaTiO₃ powder.

In the hydrothermal synthesis, an active titanium entity ([Ti(OH)₆]²⁻)having a number of hydroxyl groups and a large specific surface area isprepared, in advance, by hydrolysis of TiCl₄ or titanium alkoxide. Thisactive titanium entity is heated under a high pressure with Ba(OH)₂ inan autoclave so that barium ions (Ba²⁺) diffuse into TiO₂ to synthesizethe BaTiO₃.

In the sol-gel method, a titanium compound is allowed to directly reactwith a barium compound. A mixed solution of titanium alkoxide and bariumalkoxide may used as a starting material, or a solution of an alkoxideprecursor including titanium and barium at a ratio derived from thestoichiometry may be used as the starting material.

In the alkoxide-hydroxide route, titanium alkoxide is hydrolyzed in asolution containing Ba²⁺ to yield BaTiO₃. When titanium alkoxide ishydrolyzed in a Ba(OH)₂ solution, the alkoxide solution becomes cloudedimmediately. Specifically, TiO₂ (or [Ti(OH)₆]²⁻) is first produced inthe Ba(OH)₂ solution, and then the TiO₂ (or [Ti(OH)₆]²⁻) reacts withBa²⁺. Thus, by diffusing the Ba²⁺ into the TiO₂, as in the hydrothermalsynthesis, BaTiO₃ is prepared.

However, the wet synthesis needs to separate unreacted Ba(OH)₂ from theprecipitate, and also to extract by-produced BaCO₃ and Ba₂TiO₄ bydissolution to separate them from the BaTiO₃. Thus, the wet synthesis iscomplicated. In addition, the resulting BaTiO₃ is likely to containBa₂TiO₄, and thus the BaTiO₃ crystals could be imperfect.

Also, since the concentration of TiO₂ is low due to the use of TiO₂containing 95% water, the reaction between TiO₂ and Ba(OH)₂ does notefficiently proceed.

Furthermore, since the dissolution is performed with an acid, such asacetic acid, the resulting crystals are likely to be damaged, or Ba²⁺could be eluted to vary the mole fractions of the BaTiO₃.

In the hydrolysis method, an alkali metal, such as K or Na, is likely toadhere to the resulting BaTiO₃ because the reaction of a product fromhydrolysis of a titanium compound with a water-soluble barium salt isconducted in a strong alkaline aqueous solution of pH 13.0 or more. As aresult, impurities which cause failure in insulation resistance remainin BaTiO₃ in several hundreds of ppm even if the resulting BaTiO₃ iswashed. Therefore, such BaTiO₃ is not suitable for a dielectric ceramicmaterial which needs to be laminated.

In the pulverization method, zirconia or the like contained inpulverizing media of the ball mill or the like could be mixed intoBaTiO₃. Thus, it is difficult to prepare a pure BaTiO₃ powder. Also,since BaCO₃ and TiO₂ pulverized in this method have a relatively largeparticle size, the degree of pulverization is limited. Specifically,these materials have extensive particle size distributions and theirparticles do not grow uniformly. It is therefore difficult to make theresulting ceramic particle size small and uniform.

The hydrothermal synthesis requires large equipment and batch processes,and consequently, workability and manufacturing efficiency are degraded.Thus, the hydrothermal synthesis limits the ability to effect costreduction and increases costs.

In the sol-gel method, cubic crystalline BaTiO₃ can be obtained bycalcination at about 400° C. However, this method must be performed in adry atmosphere because barium alkoxide is liable to react with waterviolently in the atmosphere to emit smoke. Also, the alkoxides used asthe starting material are undesirably expensive.

In the alkoxide-hydroxide route, titanium alkoxide is dissolved inBa(OH)₂ to synthesize barium titanate. However, in order to completethis reaction, an excessive amount of Ba(OH)₂ is required, or NaOH orKOH must be added.

If Ba(OH)₂ is used excessively, Ba²⁺ remains in the solution after thereaction, thus making it difficult to control the composition of BaTiO₃.Also, equipment for recovering barium is necessary.

If NaOH or KOH is added, Na⁺ or K⁺ remains in the solution after thereaction, thus interfusing into the resulting BaTiO₃ to act as animpurity. Thus, it is difficult to prepare pure BaTiO₃.

Furthermore, since hydrolysis of an alkoxide by-produces an alcoholwhich is a hazardous organic solvent, explosion proof equipment isneeded, increasing costs.

SUMMARY OF THE INVENTION

Accordingly, the object of the present invention is to provide a methodfor manufacturing a high-quality complex oxide fine particle powder,such as barium titanate powder, having a high crystallinity at a lowcost, and to provide a complex oxide powder prepared by the method.

The inventors of the present invention have conducted intensive researchto achieve a fine, highly crystalline barium titanate powder by a simplemethod. As a result, it has been shown that, by fusing barium hydroxideoctahydrate (Ba(OH)₂.8H₂O) by heating, a high concentration of activebarium solution can be prepared, and that, by allowing the bariumsolution to react with titanium oxide (TiO₂) powder, a pure, fine BaTiO₃powder can be easily prepared. In addition, this method may be appliedto other complex oxides similar to BaTiO₃.

According to an aspect of the present invention, a method is providedfor preparing a complex oxide powder having a perovskite structureexpressed by the general formula ABO₃. The method includes a fusion stepof fusing a hydroxide of an element constituting the A site of thegeneral formula ABO₃ by heating. The hydroxide contains crystal water.The method also includes a reaction step of allowing the fused hydroxideto react with an oxide powder of an element constituting the B site ofthe general formula ABO₃ to yield a reaction product. The oxide powderof the B site element comprises specific ultrafine particles.

When the complex oxide powder is BaTiO₃, Ba(OH)₂.8H₂O may be used as thehydroxide and TiO₂ powder may be used as the ultrafine oxide powder.

In other words, the hydroxide may be Ba(OH)₂.8H₂O in the method of thepresent invention and the oxide powder of the B site element may be TiO₂powder. Thus, a desired BaTiO₃ powder can be prepared at a low cost.

Instead of a hydroxide containing crystal water, an anhydrous hydroxidecontaining an amount of water equivalent to the amount of the crystalwater can be used.

Accordingly, the present invention is also directed to a method forpreparing a complex oxide powder having a perovskite structure expressedby the general formula ABO₃. The method includes a dissolution step ofdissolving an anhydrous hydroxide of an element constituting the A siteof the general formula ABO₃ in a predetermined amount of water. Themethod also includes a reaction step of allowing the dissolved hydroxideto react with an oxide powder of an element constituting the B site ofthe general formula ABO₃ to yield a reaction product. The oxide powderof the B site element comprises specific ultrafine particles. The amountof water may be equivalent to the amount of crystal water contained in ahydrous hydroxide of the element constituting the A site.

When the complex oxide powder is BaTiO₃, Ba(OH)₂ may be used as thehydroxide and TiO₂ powder may be used as the ultrafine oxide powder.

In other words, the anhydrous hydroxide may be Ba(OH)₂ in the method ofthe present invention and the oxide powder of the B site element may beTiO₂ powder. Thus, a desired BaTiO₃ powder can be prepared at a lowcost.

In order to prepare an ultrafine final product, or the complex oxide,having high crystallinity, the oxide powder used as a material must beultrafine and highly crystalline.

Preferably, the ultrafine specific particles have a specific surfacearea of about 250 m²/g or more.

By using an oxide powder having a specific surface area of about 250m²/g or more, the formation of heterogeneous phases can be as suppressedas much as possible. Thus, an ultrafine, highly crystalline complexoxide powder can be prepared.

Preferably, the hydroxide and the oxide powder are weighed such that themole ratio of the A site element A to the B site element B is in therange of about 0.990 to 1.010. Thus, a pure complex oxide powder notcontaining impurities can be prepared.

The reaction step may be performed under atmospheric pressure. Thus, adesired complex oxide powder can be prepared at a low cost without usingspecial large equipment.

Preferably, the reaction step comprises a substep of applying ultrasonicwaves. Preferably, the reaction product has a specific surface area inthe range of about 60 to 100 m²/g.

By applying ultrasonic waves, a much finer reaction product having aspecific surface area of about 60 to 100 m²/g can be prepared beforecalcination.

The method may further include a calcination step of calcining thereaction product.

By calcining the reaction product, an ultrafine, highly crystallinecomplex oxide powder can be prepared.

The method may further include a dispersion step of dispersing thereaction product in a liquid and a calcination step of calcining thedispersed reaction product.

By dispersing the reaction product in a liquid and then calcining thedispersed product, the crystallographic axial ratio c/a can be furtherincreased.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block flow diagram of a method for preparing a complex oxidepowder according to an embodiment of the present invention;

FIG. 2 is an illustration showing a reaction path of BaTiO₃ synthesis ofthe present invention;

FIG. 3 is an illustration showing another reaction path of BaTiO₃synthesis of the present invention;

FIG. 4 is a transmission electron micrograph of crystals after 5 minuteshave elapsed from the start of a reaction in a reaction step ofsynthesizing BaTiO₃;

FIG. 5 is a transmission electron micrograph of crystals after 30minutes have elapsed from the start of the reaction in the reaction stepof synthesizing BaTiO₃;

FIG. 6 is a transmission electron micrograph of crystals after 60minutes have elapsed from the start of the reaction in the reaction stepof synthesizing BaTiO₃;

FIG. 7 is a scanning electron micrograph of non-calcined BaTiO₃ (driedat 200° C.);

FIG. 8 is a scanning electron micrograph of BaTiO₃ calcined at 900° C.;

FIG. 9 is a scanning electron micrograph of BaTiO₃ calcined at 950° C.;

FIG. 10 is a scanning electron micrograph of BaTiO₃ calcined at 1000°C.;

FIG. 11 is an X-ray diffraction diagram resulting from Experiment 2;

FIG. 12 is a graph showing the relationships between the particle sizeand the axial ratio c/a of BaTiO₃ powders of an Example and acomparative example;

FIG. 13 is an X-ray diffraction diagram resulting from Experiment 3; and

FIG. 14 is an X-ray diffraction diagram resulting from Experiment 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the invention, a hydroxide of an A site element is dissolved in waterat a high concentration forming a “fused hydroxide” and the solution iscombined with an oxide of a B site element and the reaction allowed toproceed. The water used can be the water of hydration of the hydroxideor an equivalent amount of water when an unhydrated hydroxide is used asa reactant. Because the amount of water generally does not significantlyexceed the amount which represents the hydration amount, the A elementconcentration in the solution is high, e.g., generally about 10 mol % ormore. The dissolution generally involves heating at a temperature whichdoes not drive the water out of the system.

-   -   The present invention will now be further illustrated with        reference to the drawings.

FIG. 1 is a block flow diagram showing steps of a method for preparingBaTiO₃ powder, according to an embodiment of the present invention.

Ba(OH)₂.8H₂O is prepared to act as an hydroxide, containing crystalwater, of an element constituting the A site of a perovskite structureexpressed by the general formula ABO₃. TiO₂ powder is also prepared toact as an oxide powder, having a specific surface area Sw of about 250m²/g or more, of an element constituting the B site of ABO₃.

The reason why the TiO₂ powder having a specific surface area of about250 m²/g or more is selected is as follows. If a TiO₂ powder having aspecific surface area of less than about 250 m²/g is used, heterogeneousphases, such as Ba₂TiO₄ and BaTi₂O₅, are formed before calcination;hence, cubic crystalline BaTiO₃ having high crystallinity cannot beobtained. The final product BaTiO₃ powder is also not tetragonalcrystalline even after calcination is performed.

Accordingly, the TiO₂ powder used in the present invention has aspecific surface area of about 250 m²/g or more in order to obtain atetragonal crystalline BaTiO₃ powder with reliability after calcination.

The crystals of TiO₂ may have a rutile structure or an anatasestructure. However, the rutile structure is likely to allow unreactedTiO₂ to remain, and therefore, an anatase-type TiO₂ is preferable.

The process of the method will now proceed to a mixing step 1. Themixing step is divided into two steps of a fusion/dissolution step 1aand a reaction step 1b.

In the fusion/dissolution step 1a, Ba(OH)₂.8H₂O and TiO₂ are weighedsuch that the mole ratio Ba/Ti of Ba to Ti of BaTiO₃to be synthesized isabout 1, and are mixed and heated in a container at a predeterminedtemperature (for example, at about 60 to 110° C.) to fuse theBa(OH)₂.8H₂O.

Since Ba(OH)₂.8H₂O contains crystal water (octahydrate), the fusion ofBa(OH)₂.8H₂O means that the Ba(OH)₂ is dissolved in the crystal water toyield a Ba solution containing a high concentration of activated Ba²⁺.In the reaction step 1b, the Ba solution is allowed to react with theTiO₂ under atmospheric pressure, and thus a reaction product, or BaTiO₃,is produced in a slurry form after a predetermined period of time (forexample, after about an hour).

In other words, the fusion/dissolution step 1a and the reaction step 1bproceed consecutively to produce a BaTiO₃ slurry from the Ba(OH)₂.8H₂Oand the TiO₂, as shown in formula (1):Ba(OH)₂.8H₂O+TiO₂→BaTiO₃+9H₂O   (1)

It is known that BaTiO₃ is synthesized through the two reaction paths(1) and (2) shown in FIGS. 2 and 3 (J. O. Eckert Jr, et al., J. Am.Ceram. Soc., 79[11], 2929-39, (1996)).

In reaction path (1), a homogeneous nucleation (a) and a heterogeneousnucleation (b) proceed in parallel to produce BaTiO₃ when TiO₂ isdissolved in the Ba solution. In reaction path (2), Ba is diffuses amongTiO₂ particles to produce BaTiO₃.

Since the solubility of TiO₂ is low and the degree of criticalsaturation of BaTiO₃ is high, the growth of BaTiO₃ particles becomesextremely slow after the nucleation of BaTiO₃. It is thereforeconsidered that reaction path (1) is predominant to reaction path (2) inan early stage of the reaction, and that reaction path (2) becomespredominant in a later stage.

However, the TiO₂ powder has a large specific surface area Sw of about250 m²/g or more, in the method of the present invention so that it canbe rapidly dissolved. As a result, a precipitate serving as seedcrystals for particle growth is sufficiently supplied and, thus,reaction path (1) substantially completes the synthesis.

A larger specific surface area of TiO₂ helps the TiO₂ dissolve; hence,as the specific surface of TiO₂ becomes larger, reaction path (1) ismore rapidly completed to produce BaTiO₃. The specific surface area ofTiO₂ is, therefore, 250 m²/g or more and there is no upper limit.

In a slurry preparation step 2, the BaTiO₃ reaction product is placed ina sand mill containing pulverizing media, and is pulverized with heatingat the above-described predetermined temperature (for example, at about60 to 110° C.) to prepare a slurry. The slurry is then recovered.

Then, the slurry was dried in a drying step 3, at a predeterminedtemperature (for example, at 200° C.) in an oven to prepare a BaTiO₃powder having an equivalent specific surface diameter of about 20 nm.

In a following calcination step 4, the resulting BaTiO₃ powder iscalcined at a temperature of about 900 to 1000° C. for about 2 hours sothat the particles grow to an equivalent specific surface diameter ofabout 100 to 350 nm. Thus, the crystal system changes to a tetragonalcrystal system to yield a highly pure, ultrafine BaTiO₃ powder.

The resulting BaTiO₃ powder has tetragonal crystals having acrystallographic axial ratio c/a of the c axis to the a axis in therange of about 1.0068 to 1.0092; hence the BaTiO₃ powder does notinclude heterogeneous phases and is highly crystalline.

The mole ratio Ba/Ti of Ba constituting A site to Ti constituting the Bsite is about 0.990 to 1.010 in the BaTiO₃ powder; hence, the BaTiO₃powder is highly pure.

In this embodiment, by preparing a high concentration of activated Basolution, a sufficiently high concentration of hydroxide ions can beobtained. It is therefore not necessary to add an alkali metal elementto prepare BaTiO₃ powder.

The method of the invention does not need to use a hazardous alkoxide orexcessive alkali, nor thus, need explosion proof equipment such as thatrequired in the alkoxide-hydroxide route. Also, since the reaction stepcan be performed at a low temperature of about 60 to 110° C. underatmospheric pressure, the manufacturing cost is reduced, without thenecessity of pressure resistant equipment required in hydrothermalsynthesis.

By allowing Ba(OH)₂.8H₂O to react with ultrafine TiO₂ powder while beingheated, an ultrafine BaTiO₃ powder containing no heterogeneous phase andhaving high crystallinity can be prepared at a low cost.

In addition, the steps of this embodiment are simple, and it istherefore easy to increases manufacturing efficiency by conducting thesteps successively. The method of the present invention makes itpossible to mass-produce a high quality BaTiO₃ powder at a low cost.

The method of the present invention is not limited to theabove-described embodiment. In the above embodiment, Ba(OH)₂.8H₂O isfused and dissolved by heating to prepare a solution of a highconcentration of Ba. However, barium hydroxide anhydride (Ba(OH)₂) maybe mixed with water in an amount equivalent to the amount of the crystalwater contained in Ba(OH)₂.8H₂O to prepare the solution of a highconcentration of Ba. Thus, a fine BaTiO₃ powder having highcrystallinity may be prepared, as in the earlier described embodiment.

Here, the BaTiO₃ reaction product is yielded in a slurry form by thereaction of the Ba solution with the TiO₂ in the reaction step 1b. Thereaction product BaTiO₃ may be further exposed to ultrasonic waveshaving a frequency of about 10 to 30 kHz while being stirred. Byapplying the ultrasonic waves, much finer cubic crystalline BaTiO₃having a specific surface area Sw of about 60 to 100 m²/g and containingno heterogeneous phase can be prepared before calcination.

The foregoing has described a method applied to the preparation ofBaTiO₃ powder in detail. However, the method can be applied to thepreparation of other complex oxides having a perovskite structure, suchas SrTiO₃ and (Ba, Ca)TiO₃.

EXAMPLES

Examples of the present invention will now be described.

Experiment 1

The inventors first investigated the reaction path of BaTiO₃ synthesis.

Anatase-type TiO₂ having a specific surface area Sw of 330 m²/g andBa(OH)₂.8H₂O were weighed such that the mole ratio of Ba to Ti in theBaTiO₃ to be synthesized is 1. Specifically, 57.342 g of TiO₂ and 202.88g of Ba(OH)₂.8H₂O were weighed. They were placed in a container heatedto 70° C. and allowed to react, and the process of BaTiO₃ production wasobserved.

FIGS. 4 to 6 are transmission electron micrographs of crystals in theprocess of BaTiO₃ synthesis.

FIG. 4 shows crystals after 5 minutes have elapsed from the start of thereaction, taken at a magnification of 790,000 times by transmissionelectron microscopy (TEM). FIGS. 5 and 6 show crystals in the regionsurrounded by the circle shown in FIG. 4 after 30 and 60 minutes haveelapsed from the start of the reaction, taken at a magnification of390,000 times by TEM.

FIGS. 4 to 6 suggest that TiO₂ powder initially having a particle sizeof about 5 nm grows to BaTiO₃ having a particle size of about 20 nmwhile the reaction proceeds.

The mole ratio Ba/Ti was measured by energy dispersive X-rayspectroscopy (EDX). As a result, the mole ratio Ba/Ti was 6/29 after 5minutes had elapsed from the start of the reaction (FIG. 4), and itchanged to 19/20 after 30 minutes had elapsed from the start of thereaction (FIG. 5), showing that the Ba and Ti molar contents weresubstantially the same.

The reaction generally proceeds through the two reaction paths shown inFIG. 2 (reaction path 1) and FIG. 3 (reaction path 2). However, theparticles in the series of the reaction shown in FIGS. 4 to 6 in thisexample were single crystals and did not exhibit diffusion of part ofthe Ba. Therefore, it is concluded that the reaction proceeded throughreaction path (1).

Experiment 2

A BaTiO₃ reaction product prepared as in Example 1 was placed in adesk-top sand mill with 800 g of partially stabilized zirconia (PSZ) of1 mm in diameter serving as pulverizing media, and was agitated andpulverized at 70° C. for 1 hour at a rotation speed of 9 s⁻¹ (540 rpm)to prepare a slurry. The slurry taken out from the mill was dried at200° C. in an oven to form a solid. The solid was divided into threeportions, and each solid was calcined at 900, 950 or 1000° C. to yieldBaTiO₃ powder of Example 1. The mole ratio Ba/Ti of Ba to Ti of theresulting BaTiO₃ powder was 0.998, according to X-ray fluorescenceanalysis (XRF).

The specific surface areas Sw of the BaTiO₃ powders of Example 1,prepared at different calcination temperatures, were measured by theBrunauer-Emmett-Teller (BET) method and the equivalent specific surfacediameters D were derived from the specific surface areas. The powders ofExample 1 were subjected to powder X-ray diffraction (XRD), and theaxial ratio c/a of the c axis to the a axis of the crystals wascalculated based on the XRD results. For the sake of comparison,non-calcined powder dried at 200° C. was also subjected to the samemeasurement.

Table 1 shows the results.

TABLE 1 Equivalent Calcination Specific specific surface TemperatureMole ratio Axial ratio surface area Sw diameter D (° C.) A site/B sitec/a (m²/g) (nm) non-calcined 0.998 1.0000 47.61 21.0 (dried at 200° C.)900 0.998 1.0075 7.31 137 950 0.998 1.0082 5.69 176 1000  0.998 1.00883.88 258

As shown in Table 1, the non-calcined powder exhibited a specificsurface area Sw of 47.61 m²/g, a small equivalent specific surfacediameter D of 21.0 nm, and a axial ratio c/a of 1.000, indicating acubic system.

Table 1 also suggests that the particle size is increased by calcinationand that, as the particle size becomes larger, the axial ratio c/aincreases so that the crystal system changes from a cubic system to atetragonal system.

FIGS. 7 to 10 are scanning electron micrographs of the non-calcinedpowder and powders of Example 1 calcined at 900, 950 or 100°C. They showthat, as the calcination temperature becomes higher, the particles growto be larger.

FIG. 11 shows the results of XRD. The horizontal axis represents thediffraction angle 2θ and the vertical axis represents intensity(arbitrary unit). In FIG. 11, the white triangles represent BaCO₃ whichcould be by-produced in the reaction, and the black squares representBa₂TiO₄ having a Ba content higher than that of BaTiO₃.

As shown in FIG. 11, even the non-calcined powder exhibits a peak ofcubic crystalline BaTiO₃. It is therefore shown that cubic crystallineBaTiO3 having high crystallinity can be produced even by synthesis at alow temperature of 70° C., and that heterogeneous phases, such as BaCO₃and Ba₂TiO₄, are hardly by-produced.

Thus, ultrafine, cubic crystalline BaTiO₃ having high crystallinity canbe produced before calcination, as distinct from the known methods suchas the pulverization method. Therefore, ultrafine, tetragonalcrystalline BaTiO₃ powder can be obtained by calcination.

BaTiO₃ powders of Comparative Examples 1 and 2 were prepared by thehydrothermal synthesis and the alkoxide-hydroxide route, respectively.The relationship between the particle size and the axial ratio c/a ofthe BaTiO₃ powders of Example 1 and Comparative Examples 1 and 2 wereinvestigated.

The BaTiO₃ powder of Comparative Example 1 was prepared by the followingprocedure.

In 10 mL of water, 0.01 mol of NaOH was dissolved to prepare a NaOHsolution, and 2.5 mL of BaCl₂ or Ba(OH)₂ and 2.5 mL of TiO₂ were addedto the NaOH solution. The mixture was heated in an autoclave to reactunder conditions of a high temperature of 250° C. and a high pressure of5 MPa. The resulting precipitate was filtered and washed with water.

The reaction product was subjected to XRD and unreacted TiO₂ and BaCO₃were not detected.

The reaction product was allowed to react with BaCl₂ for one week toprepare a tetragonal crystalline BaTiO₃ having a particle size of 50 nm.Then, the BaTiO₃ was divided into three portions, and each was calcinedat 900, 950 or 1000° C. to yield a BaTiO₃ powder of Comparative Example1.

The BaTiO₃ powder of Comparative Example 2 was prepared by the followingprocedure.

Into a solution of 2.5×10³ mol/m³ titanium isopropoxide (Ti[OCH(CH₃)₂]₄)maintained at a temperature of 60° C., Ba(OH)₂.8H₂O powder was addedsuch that the mole ratio Ba/Ti of Ba to Ti of BaTiO₃ to be synthesizedis 1, and was strongly stirred. After the Ba(OH)₂ was uniformlydissolved and then 3 minutes elapsed, the solution became clouded andgelled. The resulting gelled slurry was subjected to infraredspectroscopy. As a result, absorption bands were observed at 380 cm ⁻¹and 570 cm⁻¹; hence, it was shown that a substance having a perovskitestructure was present in the gelled slurry.

The gelled slurry was dried in an oven heated to 150° C. for 2 hours toyield a BaTiO₃ powder having a particle size of 48 nm. This BaTiO₃powder had a mole ratio Ba/Ti of 0.998 and a specific surface area Sw of45 m²/g, according to XRF. The BaTiO₃ powder was divided into threeparts, and each was calcined at 900, 950 or 1000° C. to yield BaTiO₃powder of Comparative Example 2.

FIG. 12 shows the relationship between the particle size and the axialratio of the calcined BaTiO₃ of Example 1 and Comparative Examples 1 and2. The white circles, black rhombus, and white squares shown in FIG. 12represent Example 1, Comparative Example 1 and Comparative Example 2,respectively.

FIG. 12 shows that BaTiO₃ of Example 1 has an larger axial ratio c/a anda smaller particle size than those of Comparative Example 1, a cubicsystem having high crystallinity.

The BaTiO₃ of Comparative Example 2 by the alkoxide-hydroxide route hadcrystallinity higher than that of Example 1. However, thealkoxide-hydroxide route needs a molar amount of Ba(OH)₂ which was 2 to3 times larger than that of TiO₂, and thus needs such an excessiveamount of Ba(OH)₂.

In contrast, the molar amounts of TiO₂ and Ba(OH)₂ used in Example 1 aresubstantially the same. It is not necessary to use an excessive amountof Ba(OH)₂, and therefore Ba²⁺ does not remain in the solution after thesynthesis. Thus, it is easy to control the composition of BaTiO₃ and toreduce manufacturing costs.

Experiment 3

BaTiO₃ powders were prepared using TiO₂ powders having differentspecific surface areas Sw according to the following procedure, andsubjected to XRD to investigate whether a heterogeneous phase waspresent.

Anatase-type TiO₂ powder having a specific surface area of 250 m²/g inan amount of 62.410 g and Ba(OH)₂.8H₂O in an amount of 119.77 g wereweighed so that the mole ratio Ba/Ti of Ba to Ti of BaTiO₃ to besynthesized would be 1.

The weighed materials were placed in a desk-top sand mill with 800 g ofPSZ of 1 mm in diameter and agitated and pulverized at 70° C. for 1 hourat a rotation speed of 9 s⁻¹ (540 rpm) to prepare a slurry, as inExperiment 2. The slurry taken out from the mill was dried at 200° C. inan oven to yield BaTiO₃ of Example 11 in a solid form.

In the same manner, anatase-type TiO₂ powder having a specific surfacearea of 300 m²/g in an amount of 189.28 g and Ba(OH)₂.8H₂O in an amountof 56.898 g were weighed so that the mole ratio Ba/Ti of Ba to Ti ofBaTiO₃ to be synthesized would be 1. Thus, BaTiO₃ of Example 12 wasprepared in a solid form, as in Example 11.

In the same manner, anatase-type TiO₂ powder having a specific surfacearea of 330 m²/g in an amount of 202.01 g and Ba(OH)₂.8H₂O in an amountof 56.898 g were weighed so that the mole ratio Ba/Ti of Ba to Ti ofBaTiO₃ to be synthesized would be 1. Thus, BaTiO₃ of Example 13 wasprepared in a solid form, as in Example 11.

Furthermore, in the same manner, anatase-type TiO₂ powder having aspecific surface area of 240 m²/g in an amount of 75.721 g andBa(OH)₂.8H₂O in an amount of 270.51 g were weighed so that the moleratio Ba/Ti of Ba to Ti of BaTiO₃ to be synthesized would be 1. Thus,BaTiO₃ of Comparative Example 11 was prepared in a solid form, as inExample 11.

The BaTiO₃ of Examples 11 to 13 and Comparative Example 11 weresubjected to XRD.

FIG. 13 shows the results of XRD. The horizontal axis represents thediffraction angle 2θ and the vertical axis represents intensity(arbitrary unit). The white triangles, black squares and white squaresrepresent heterogeneous phases of BaCO₃, Ba₂TiO₄ and BaTi₂O₅,respectively. For example, Comparative Example 11 exhibits two sharppeaks at diffraction angles 2θ of 25° and 28° with white squaresthereabove, which show the formation of heterogeneous phases of BaTi₂O₅.Comparative Example 11 also exhibits a sharp peak at a diffraction angle2θ of 29° with a black square thereabove, which shows the formation of aheterogeneous phase of Ba₂TiO₄.

As shown in FIG. 13, while the BaTiO₃ of Example 11 (specific surfacearea Sw: 240 m²/g) includes heterogeneous phases of BaCO₃, BaTi₂O₅ andBa₂TiO₄ between a wide rage of diffraction angles 2θ of 20° to 60°, theExamples 11 to 13 (specific surface areas Sw: 250 to 330 m²/g) includefew heterogeneous phases of BaCO₃ and no BaTi₂O₅ or Ba₂TiO₄. Thus, it isshown that TiO₂ powder must have a specific surface area of at leastabout 250 m²/g.

Experiment 4

The BaTiO₃ reaction product slurry was exposed to ultrasonic waves, andproperties of the resulting powder was investigated.

First, 150 mL of water was placed in a vessel heated by circulatingvapor of 100° C. to prepare hot water of 60° C. Anatase-type TiO₂ havinga specific surface area of 300 m²/g in an amount of 98.067 g and Ba(OH)₂in an amount of 185.318 g were weighed so that the mole ratio Bi/Ti ofBa to Ti of BaTiO₃ to be synthesized would be 1.003, and were then addedto the hot water. The Ba(OH)₂ was allowed to start reacting, with heatgenerated, to prepare BaTiO₃ in a slurry form.

The BaTiO₃ was stirred with a magnetic stirrer while being exposed toultrasonic waves having a frequency of 16 kHz by immersing in the hornof an ultrasonic emitter.

Then, the BaTiO₃ was pulverized with a desk-top sand mill, insubstantially the same manner as Experiment 2, to prepare a slurry. Theslurry was taken out of the mill and placed in an oven heated to 200° C.to dry. Thus, a non-calcined solid BaTiO₃ was prepared.

The mole ratio Ba/Ti of Ba to Ti of the resulting non-calcined BaTiO₃was 0.9998, according to XRF analysis.

The non-calcined BaTiO₃ was subjected to XRD to investigate whether aheterogeneous phase was present.

FIG. 14 shows the results of XRD. The horizontal axis represents thediffraction angle 2θ and the vertical axis represents intensity(arbitrary unit).

As shown FIG. 14, the non-calcined BaTiO₃ does not include anyheterogeneous phases such as Ba₂TiO₄, and it is therefore shown that apure, solid BaTiO₃ can be prepared before calcination.

The axial ratio c/a of the non-calcined solid BaTiO₃ was calculated fromthe results of XRD, as in Experiment 2. Also, the specific surface areaSw of the BaTiO₃ was measured by the BET method and the equivalentspecific surface diameter D was derived from the specific surface areaSw.

Table 2 shows the results.

TABLE 2 Equivalent Specific specific surface Mole ratio Axial ratiosurface area diameter D A site/B site c/a Sw (m²/g) (nm) BaTiO₃ 0.99981.0000 64.08 15.6 dried at 200° C.

Table 2 shows that the non-calcined BaTiO₃ dried at 200° C. afterexposure of ultrasonic waves has a specific surface area Sw of 64.08m²/g and an equivalent specific surface diameter D of 15.6 nm, smallerthan that of the non-calcined BaTiO₃ in Experiment 2, shown in Table 1.It is therefore shown that the exposure to ultrasonic waves contributesto the reduction of the particle size of BaTiO₃. Also, the axial ratioc/a is 1.000, showing a cubic system.

Experiment 5

A BaTiO₃ dried in an oven heated to 200° C., as in Experiment 2, wasplaced in a ball mill with isopropyl alcohol, and was wet-pulverized for10 hours to be dispersed in the isopropyl alcohol. The dispersed BaTiO₃was dried at 200° C. in an oven to form a solid. The solid was dividedinto three portions, and each solid was calcined at 900, 950 or 1000° C.for 2 hours to yield a calcined BaTiO₃ powder.

The axial ratio c/a, the specific surface area Sw, and the equivalentspecific surface diameter D of the resulting calcined BaTiO₃ powders andthe non-calcined BaTiO₃ at dried at 200° C. were measured.

Table 3 shows the results.

TABLE 3 Equivalent Calcination Axial specific specific surfaceTemperature Mole ratio ratio surface area diameter D (° C.) A site/Bsite c/a Sw (m²/g) (nm) non-calcined 0.998 1.0000 55.2 18.1 (dried at200° C.) 900 0.998 1.0080 7.53 133 950 0.998 1.0092 4.90 204 1000  0.9981.0098 3.25 308

Table 3 shows that by calcining at a temperature of 950° C. or more, thespecific Sw is reduced, but the axial ratio c/a can be increased incomparison with experiment 2, shown in Table 1, in which the dispersionstep was not performed.

1. A method for preparing a complex oxide powder having a perovskitestructure expressed by the general formula ABO₃, the method comprisingdissolving and heating a hydroxide of an element constituting the A siteof the general formula ABO₃ in water to form a fused hydroxide, thewater consisting of crystal water contained in the hydroxide of theelement constituting the A site of the general formula ABO₃; reactingthe fused hydroxide with an oxide powder of an element constituting theB site of the general formula ABO₃, the B site oxide powder beingparticles having a specific surface area of about 250 m²/g or more, toform a reaction product comprising the complex oxide powder in the formof ultrafine particles; and calcining the reaction product.
 2. A methodfor preparing a complex oxide powder according to claim 1, wherein thehydroxide is barium hydroxide octahydrate and the oxide powder of the Bsite element is titanium oxide powder.
 3. A method for preparing acomplex oxide powder according to claim 2, wherein the reaction furthercomprises ultrasonication.
 4. A method for preparing a complex oxidepowder according to claim 1, wherein the hydroxide of an elementconstituting the A site and the oxide powder of the element constitutingthe B site are weighed such that the mole ratio of the A-site element tothe B-site element is in the range of about 0.990 to 1.010.
 5. A methodfor preparing a complex oxide powder according to claim 1, wherein thereaction is performed under atmospheric pressure.
 6. A method forpreparing a complex oxide powder according to claim 1, wherein thereaction further comprises ultrasonication.
 7. A method for preparing acomplex oxide powder according to claim 6, wherein the reaction producthas a specific surface area in the range of about 60 to 100 m²/g.
 8. Amethod for preparing a complex oxide powder according to claim 1,further comprising dispersing the reaction product in a liquid.
 9. Amethod for preparing a complex oxide powder according to claim 1,wherein the oxide of the B site element is anatase TiO₂.